Modern genetics is a development of discoveries made by the abbot Mendel in the sixties of the last century, discoveries whose significance was partly perceived by himself and by no one else for another thirty years. The science of biology has thrown a considerable amount of light upon the nature and the problems of human societies and it certainly throw a great deal more light upon them. Moreover, there is likely to be a measure of assortative mating, since men and women usually marry within their social class, and often marry those of similar type to them. There is a process of social segregation tending to emphasize the character of the groups. But the complexities, both of biological development and social structure, are too great for this process to be rapid or uniform. Theoretical considerations lead us to expect some such result as the intelligence testing seems to indicate in the case of the social classes.